1. Field of the Invention:
This invention relates to treating oil and gas wells. More specifically, this invention relates to a method of preventing the precipitation of ferrous sulfide and sulfur during acidizing of sour wells.
2. Description of the Prior Art:
It is generally known and an accepted commercial practice to stimulate oil and gas production by the use of various aqueous mineral acid solutions and the like. Such acidizing treatments are commonplace in the oil and gas industry and are frequently employed in both production and injection wells. As such, various types of acidizing media with or without specific additives intended to alleviate various specific problems have been historically proposed and employed. For example, formation damage from ferric hydroxide precipitation is a potential problem during any acidizing treatment. Acid readily dissolves iron scale in pipe and equipment, and attacks iron containing minerals present in the formation. This dissolved iron remains in solution until the acid spends. As the pH rises above two that portion of the iron present in the ferric, Fe(III), oxidation state precipitates as gelatinous ferric hydroxide. This can cause serious formation damage.
Iron dissolved during an acidizing treatment can exist in either ferric, Fe(III), or ferrous, Fe(II), oxidation state. Upon spending of the acid, the iron present in solution in the Fe(III) form will beging to precipitate at a pH of about 2.2. Upon reaching a pH of 3.2, essentially all of the dissolved fe(III) will have precipitated unless an effective iron stabilizer is used. In contrast, Fe(II) hydroxide will not precipitate below a pH of 7.7. Since spent acid ususally reaches a maximum pH of about 5.3, precipitation of the Fe(II) hydroxide is rarely a problem. Consequently, one needs only to be concerned about the stabilizing of the Fe(III) iron dissolved in the acid.
Previous studies have shown that most iron, dissolved by treating acid, is present in the Fe(II) oxidation state. It has been estimated that, on the average, the Fe(II) to Fe(III) ratio of spent acid is about 5:1. This varies greatly, however, depending upon well conditions and the type of formation being treated.
It is generally known that ferric hydroxide precipitation can be prevented by adding iron stabilizers to the acid. Iron stabilizing agents have been used since the early years of acidizing to prevent precipitation of ferric hydroxide from spent acid solutions. Iron sequestering agents as well as reducing agents have been proposed for control of this problem (see, for example, U.S. Pat. Nos. 2,175,081 and 2,175,095). Sequestering agents most commonly used include citric acid, acetic acid, mixtures of citric and acetic acids and ethylenediaminetetraacetic acid (EDTA). Reducing agents most commonly proposed as iron stabilizers were sulfur compounds, such as sulfurous acid or materials which generate sulfurous acid upon addition to HCl. These agents function by converting any ferric ion present to the non-damaging ferrous state. These materials have found limited applications due to problems associated with their use. For example, sulfurous acid reduces Fe(III) as follows: EQU H.sub.2 SO.sub.3 +2FeCl.sub.3 +H.sub.2 O.fwdarw.2HCl+2FeCl.sub.2 +H.sub.2 SO.sub.4 ( 1)
Although Fe(III) is reduced to Fe(II), sulfuric acid is formed in the process resulting in precipitation of CaSO.sub.4 upon spending of the acid as follows: EQU H.sub.2 SO.sub.4 +CaCO.sub.3 .fwdarw.CaSO.sub.4 .dwnarw.+CO.sub.2 +H.sub.2 O (2)
In addition, SO.sub.2 fumes liberated from the sulfuric acid are very irritating making handling of the overall system rather unpleasant.
In addition to the ferrous hydroxide precipitation problem, the present inventor has discovered that other precipitates can form during the acidizing of a sour gas well (as explained later). It has been discovered that the additional precipitates in the case of sour wells are not adequately accounted for by the prior art sequestering and reducing agents, but rather require a specific combination of an iron complexing agent and an iron reducing agent to alleviate and prevent formation damage.